You found a XK for $2500? And it runs? Some of the well used modern sedans might come in at that price but most have fuel injection, knock sensors and different designed combustion chambers that will tolerate the use of lower octane gas. I guess my main comment is if you can afford a Jaguar, then you can afford to take care of it per factory recommendations. If having to run regular helps you afford it, then hopefully, you will not have any major breakdowns.
Oh, no. We‘re on the #xk-engine list.
Martins 4.2 S2 E runs just as „well“ on 95 though.
I have the money but there are reasons for that. A good E would be set up for premium and see only that.
my 9:1 (with unknown head milling) wants 98 Ron, or I can detect pinging at WOT when ambient is high, with timing at 10btdc. If I were to use 95, wouldnt feel comfortable using WOT, which is no way to drive a Jag, even an old one, so I just pay at the bowser
On the other hand, I cant understand why guys with EFI cars, if specified to run on 91 would waste their money, as it doesnt make more power
A little, yes, but only when it’s hot so I avoid long WOT and always back off should it sound wrong. Apart from that I never feathered it either, I just think that if I‘m just a little mindful I can have the same enjoyment for a few bucks less and resort to premium on hot summer days. There are other things that lower my enjoyment, and I still love driving.
There was this guy with his brand new VW gti who only ever used 102. to get to work. Never did he not complain about the cost; I‘d rather have 30 hp less and be happy with it except for some occasions.
Back to the topic, you say that if it runs with 91 one should use 91?
I would never use 91 Ron in a 9:1 XJ engine except in an emergency
In the XJ, never!
In a modern car that is specified to do so. Our lowest grade is 95 but I wouldn’t do 91 if 95 is pushing it. Of course not!
sorry my misunderstanding,
yes 91 in vehicles specified for it saves money, the RON rating specifies knock resistance and no power will be gained by running higher octane.
My Jeep Cherokee has 350K, proven no mileage benefit in running higher octane fuel, takes 91
That is interesting. Why wouldn’t they set it up for, theoretically, 120 if it has knock sensors?
I assume you can save a few percent with more advance…
Hi Dick:
Can you give me your thoughts on advance. I have the initial advance set at 10 Degrees BTDC, No preignition or pinging I can detect with the engine under load in our hot summer weather. I use premium; 91 octane. Is there any advantage to increasing the initial advance as long as I don’t induce pinging?
I realize I may be asking for a simple answer to a complicated question.
My Jaguar XJ6 workshop manual (1972) recommends 98 RON for the high compression 4.2 engine and 94 RON for USA /CANADA markets. I can’t see if the recommended timing for UK v US/CANADA markets is different. Why the recommended octane rating difference?
Frankie
I am always surprised when people cite static timing. I couldn’t care less how my engine runs with no load. The only use these days for a static figure is to start the engine after a rebuild so that you can set the timing with a light at full advance. Your real timing could be miles out
Due to age and advance spec.
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Agree her track test should have been blind.
For my modern car I’m just looking at hp/$ and the winner is ASDA…
Shell 166.1
Esso 167.0
BP ult. 161.0
BP std. 169.9
ASDA 173.4
My Ford is designed to take advantage of E85 (85% ethanol) and it is around .10-.15 cents cheaper so I tested a tank. I normally get 17mi/ga. with the E85 it dropped to 12mi/ga.!!! Hardly worth it.
Some day for my Jag I will use the full non-corn gas available at my Buc-ee’s but I believe it is rated at 93 RON
At my altitude (5280’ -1609 m), that is all I ran in Tweety.
With the Mallory, I had to back the timing off a bit, but with the EDIS, I could run a more aggressive advance. ~25k miles, never a problem.
At sea level? That’s a different story.
Peter:
I’ve checked the advance at the speeds cited in the manual and my distributor gives me the advance numbers the manual says I should see. This with the initial advance set at 10 degrees BTDC.
Am I wrong in assuming if I bump up the initial advance 2 degrees I will also change the timing by 2 degrees across the entire RPM range?
AFAIK the octane rating is approximately the same, i.e. 98 UK RON = 94 US/Can RON
Just a different method (UK/NA) of calculating the RON values (how much kerosene tolulene etc is added to regular to make premium)
Right. If you’ve strobed it you’re fine
Yes, 2 degrees would be added to all points. The old numbers were for old gas spec
Changing the timing by turning the distributor will change it that amount at all rev ranges compared to the previous setting. I see a lot of people with Series 2 cars (dual ZS carbs) change to triple SUs and not change the distributor. The static timing and advance curve for the cars is totally different. The Emission distributor has a 20 degree advance curve which is 40 at the crank and then add 5 for static and you have 45 degrees. If you set it at 10 BTDC as the SU setups call for, you have 50 degrees. This is piston burning territory. The Series 1 4.2 triple carb setup uses a 11 degree which with static timing is 32 degrees at full advance. The Vacuum advance on the early distributors only come into play with light throttle load which the engine can handle. I have a fully functional SUN distributor machine set up for the Lucas distributors. Very rarely are they advancing to factory specs. Point plates loose, worn lobes, springs sprung or replaced with wrong ones. Overall, hard to advise just advancing the timing. On my race XKE, I ran initial timing at 17 degrees but had a short advance curve for a total of 37 degrees all in. Every engine is different and compression, carb type, cams,& exhaust. If in doubt, set it up as the factory did. Match the distributor and timing to the carb type. If you are in doubt as to the integrity of the distributor, get it checked. You will not hear high speed preignition but it happens and can damage pistons and upper rings. I have seen engines come apart with all 6 upper rings in pieces. Timing to far advanced or low octane fuel would be the best guess as to what caused it.
A common misconception: pre-ignition is not synonymous with detonation.
Correct. And neither are good to have in an engine except for controlled detonation as it is designed for.
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Excellent information, thanks Dick.
That’s what I got when I checked mine. I have the two Strombergs with the Joe Curto adjustable jets and a Petronix distributor and 10 degrees static timing. I’m thinking perhaps I should leave well enough alone.